The present invention relates to the use of photosensitive media to store images and specifically to the use of a particular doped silver halide-tin oxide medium to store positive images.
The use of evaporated binder-free silver halide layers as photographic media has long been known. A good summary of the patent literature in this field is found in the U.S. Defensive Publication of Maskasky, T966,003 of Jan. 3, 1978. U.S. Pat. Nos. 2,945,771 to Mansfeld and 3,219,448 to LuValle et al. describe methods by which such films may be deposited on substrates such as glass or plastic, while a detailed discussion of the deposition, processing and performance of evaporated silver bromide films is provided by A. Shepp et al. in "Evaporated Silver Bromide as a Photographic Recording Medium", Photographic Science and Engineering, 11, (5), pp. 316-321 (1967).
U.S. Pat. Nos. 3,219,452 to Hartouni, 3,368,895 to Matejic et al., and 3,658,540 to Malinowski describe materials and/or methods which have been employed to sensitize such photographic films, in order to enhance the latent image formation or chemical developability thereof. These patents also describe techniques for achieving positive images. In general, binder-free photographic films of the kind described in the above-cited literature are adapted for use in a conventional photographic mode, i.e., a mode wherein the steps of latent image formation by exposure to light and image development by chemical means are required to provide an image of suitable optical density.
Binder-free silver halide based compositions have also been employed in photochromic films, which are films typically exhibiting the properties of visible darkening on exposure to actinic radiation (e.g., short wavelength visible or ultraviolet light) and fading to the original state in the absence thereof. U.S. Pat. No. 3,512,869 to Plumat et al. describes photochromic films incorporating evaporated silver halides or the halides of other metals, which films darken in sunlight and fade in darkness. These films may be catalyzed with copper, cadmium or nickel halides to make them more sensitive to yellow or red visible light, if desired. U.S. Pat. No. 3,875,321 to Gliemeroth and French Patent No. 2,236,196 are additional patents disclosing reversibly darkenable photochromic films, while in the Soviet Journal of Optical Technology, pp. 117-118 (February 1972), A. F. Perveyev et al. describe AgCl-CuCl photochromic coatings.
Generally, rapid darkening in the presence of light and fast fading in the absence thereof are the properties most desired in photochromic films. Hence such films are not suitable photographic media because they do not provide a permanent record of the darkening or fading processes.
A photosensitive medium which could photographically preserve images without the use of chemical developers would offer obvious processing advantages, particularly if it would provide a positive image of the photographed subject matter. One medium recently developed for optical information storage applications, described in our copending, commonly assigned U.S. patent application Ser. No. 86,690, filed Oct. 22, 1979, can be directly darkened by exposure to ultraviolet light to provide a good contrast negative image. That medium comprises binder-free tin oxide and doped silver chloride, deposited by thermal evaporation as a mixture or as alternating layers of tin oxide and doped silver chloride, wherein the silver chloride is doped with cadmium chloride and, optionally, copper chloride.
When provided as a thin film on a suitable substrate this medium provides very high resolution as needed for digital information storage applications. However, the information is stored as a negative rather than a positive image.